(1) Field of the Invention
This invention relates to a liquid crystal panel and a method of manufacturing the same, and more particularly to a liquid crystal panel having a structure in which recesses formed at respective locations of contact holes each for making contact between a pixel electrode on a TFT (Thin Film Transistor) substrate and a TFT are filled with a resin or the like, and a method of manufacturing the same.
(2) Description of the Related Art
A liquid crystal panel mounted in a liquid crystal display device has a structure in which a liquid crystal layer is sandwiched between a TFT substrate mainly formed with pixel electrodes, TFTs for driving pixel electrodes, and bus lines, and a counter substrate, such as a CF substrate, mainly formed with pixel electrodes and color filters (CFs), with polarizing plates bonded to the substrates. Recently, a liquid crystal panel of this kind is produced by forming a flatting layer of a resin having a thickness of approximately 4 μm to shield the influence of a lateral electric field due to bus lines, thereby realizing a high aperture ratio of the panel.
The flatting layer is formed with deep contact holes each for making contact between a pixel electrode and a TFT. After the TFTs and the flatting layer are formed in the mentioned order, the contact holes reaching the TFTs are formed. Then, a transparent conductive film layer, forming a pixel electrode, is formed on a surface of the flatting layer, a wall surface of each contact hole, and a drain electrode of the associated TFT, whereby the pixel electrode and the TFT are caused to make contact with each other. This makes the depth of the contact holes equal to the thickness of the flatting layer, and after formation of the pixel electrodes, relatively deep recesses remain as formed in the TFT substrate at the locations of the contact holes. It is preferable that the recesses are formed to have a minimized diameter so as to prevent the aperture ratio of the liquid crystal panel from being lowered. Normally, the contact holes are formed to have a diameter of approximately 5 μm.
As described above, the small and deep contact holes having a diameter of 5 μm and a depth of 4 μm are formed in the TFT substrate, and after the pixel electrodes are formed in the contact holes, the recesses are formed at the respective locations of the contact holes. It is difficult for the liquid crystal filled between the recess and the CF substrate to enter the recesses. As a consequence, some of these recesses are left behind as vacuum areas, or gasses in the recesses are moved to the liquid crystal layer to thereby cause faulty display of the liquid crystal panel. Further, when strong pressure is partially applied to the liquid crystal panel from outside, the cell gap of a liquid crystal cell is changed to temporarily generate vacuum areas and air bubbles, but these vacuum areas and air bubbles are normally extinguished as soon as the application of the pressure stops. However, if small and deep contact holes as described above exist, the air bubbles can remain in the recesses, whereby faulty display of the liquid crystal panel can be caused.
To cope with the above problems, there have been conventionally proposed various methods, including a method of filling recesses remaining at respective locations of contact holes after formation of pixel electrodes, with an insulating film, a method of forming columnar spacers in the recesses so as to maintain the cell gap (see e.g. Japanese Unexamined Patent Publication (Kokai) No. H09-304793 and No. 2002-169166).
However, to fill recesses remaining at the locations of contact holes after formation of pixel electrodes in the contact holes, with an insulating film, it is necessary to provide a photolithography step of forming the insulating film, and further a flatting step of flatting the formed insulating film, which causes increases in the number of steps and costs.
Further, if columnar spacers for maintaining the cell gap are formed in recesses existing in all the pixel areas, the degree of freedom of the positional relationship between the TFT substrate and the CF substrate is lost to lose flexibility to an impact, whereby the liquid crystal panel is made trouble-prone. Further, since the degree of freedom of the positional relationship between the TFT substrate and the CF substrate is lost, vacuum areas and air bubbles tend to occur in the liquid crystal panel by a volume change of liquid crystal caused by a change in environmental temperature conditions, whereby faulty display of the liquid crystal panel is made liable to occur. To overcome this problem, the columnar spacers are arranged at predetermined space intervals such that optimum density of columnar layers is attained in the liquid crystal panel. In this case, however, bubbles can still occur in recesses having no columnar spacers formed therein.
Further, if the step of filling recesses and the step of forming columnar spacers are carried out as separate steps, the number of steps required for manufacturing the liquid crystal panel is increased, whereby productivity thereof is largely reduced.